There are several problems relating to the systems using charged particle beams, such as an electron microscope and an electron beam lithography system. Each system internally has a beam source for generating a charged particle beam such as an electron beam or ion beam, thin down the charged particle beam into a smaller diameter by using electromagnetic lenses and the like, and irradiate the charged particle beam onto a target. One problem is that there is the necessity for periodic maintenance due to contamination of the inner wall of a beam duct in which the charged particle beam is to travel. Other elements of the system also may need periodic decontamination.
The charged particle beam is irradiated onto a target using electromagnetically formed electron optics or ion beam optics (charged particle optics), and the duct in which the charged particle beam is to travel needs to be maintained in a vacuum to ensure a high mean free pass for the charged particle beam. For this reason, the components of the charged particle optics, such as electromagnetic lenses, permanent magnet lenses, electric/magnetic deflectors, and elements having apertures, are arranged on the periphery of, or partly inside, a vacuum chamber.
Although this vacuum vessel is maintained in a vacuum during the operation of the system using charged particle beams, the performance limits of vacuum pumps make it difficult to completely remove oil-containing or other residual gases or degas the charged particle beam target. Continued operation of the system causes a highly reactive part of molecules (radicals) or the like due to interactions between the residual gases and the charged particles, and thus contaminates the inner wall of the vacuum vessel and other internal structures of the system. The effects of contamination with the hydrocarbon heavily contained in the residual gases are particularly significant, and the hydrocarbon causes the contamination to accumulate on the internal components of the vessel.
The charged particle beam has a charge and is therefore deflected by an electromagnetic field. However, the charged particle beam is significantly affected by internal contamination of the vacuum vessel, in particular. This event is considered to be due primarily to the fact that the formation of insulation films on the surfaces of metallic components due to the contamination produces a residual charge due to a recoil electron or the like and thus causes a local electric field to be formed. To stabilize the course of the charged particle beam and maintain system performance, therefore, it is essential to maintain the cleanliness of the elements having apertures of the system, the elements having apertures periphery, and other sections, by cleaning each of these elements periodically or replacing parts. Such maintenance is particularly important for the system internal structures disposed at the positions near the beam duct where recoil electrons concentrate.
To clean the system interior, however, it is necessary in most cases to stop the vacuum pumps and then introduce the atmosphere into the system before disassembling it, and for this reason, normal operation of the system needs to be stopped. It is desirable that the use of such a procedure be avoidable, since this procedure not only requires great amounts of expenses, but also incites a decrease in productivity due to a decrease in system availability according to the particular shutdown period of the system. A system that allows internal contamination to be removed by introducing a gas without disassembling the system (refer to, e.g., Japanese Patent Laid-open No. 9-245716), and a system that retards contamination by forming a catalyst on an inner wall (refer to, e.g., Japanese Patent Laid-open No. 2002-248338), are known as conventional systems using charged particle beams. However, if the frequency of system cleaning can be reduced by examining the degree of contamination, great contributions can be made to the improvement of productivity.